1. Field of the Invention
This invention relates to thixotropic cement compositions and methods of using the same such as in cementing pipe strings in well bores without substantial loss of the cement composition.
2. Description of the Prior Art
Thixotropic cement compositions have been used heretofore for plugging lost circulation zones in well bores and for cementing pipe strings in well bores containing fractures, vugs and other high permeability zones. The gel strengths of the thixotropic cement compositions are low during mixing and pumping when the compositions are subjected to high shear stress, but when the compositions are subjected to static conditions, i.e., low or no shear stress, they quickly develop high gel strengths and high viscosities. This property prevents substantial loss of the thixotropic compositions when they are placed across and enter lost circulation zones.
The thixotropic cement compositions which have been utilized heretofore often include solid particulate thixotropy imparting agents which must be mixed with dry particulate cement and other solid additives prior to forming the solid materials into a slurry by the addition of mixing water thereto. In offshore operations, the particulate solid materials are dry blended onshore and several blends are taken to the offshore rig, e.g., thixotropic blends and non-thixotropic blends, so that the appropriate blend may be selected and mixed with water just before the cementing job is performed. This procedure of having several dry particulate solid cement blends on the site lends itself to confusion and possible mistake and also creates the requirement that the unused dry particulate blend must be returned or disposed of.
In well cementing operations such as primary cementing whereby pipe strings are sealingly cemented in well bores, thixotropic well cementing compositions and methods which prevent substantial loss of the cement compositions into fractures, vugs and the like in the well bore are advantageous. Since the cement composition is circulated downwardly through the pipe string to be cemented and upwardly into and through the annulus between the pipe string and the walls of the well bore, it would also be advantageous to utilize a delayed thixotropic cement composition which does not develop thixotropic properties, and as a result is less viscous and more easily pumped, until it has been pumped a substantial distance through the pipe string.
Thus, there are continuing needs for improved thixotropic cement compositions whereby the thixotropy imparting agent can be dissolved in the mixing water instead of being added to the dry particulate solid cement and whereby the onset of thixotropic properties after the cement composition has been prepared is delayed.
The present invention provides delayed thixotropic cement compositions and methods which meet the needs described above and overcome the deficiencies of the prior art. The delayed thixotropic well cementing compositions of the present invention are basically comprised of a hydraulic cement, sufficient water to form a slurry and a water soluble delayed thixotropy imparting additive selected from the group consisting of carbonic acid and alkali metal carbonates.
The methods of this invention for cementing in a subterranean zone penetrated by a well bore are basically comprised of the following steps. A delayed thixotropic cement composition comprised of a hydraulic cement, sufficient water to form a slurry and a water soluble delayed thixotropy imparting additive selected from the group consisting of carbonic acid and alkali metal carbonates is prepared. Thereafter, the cement composition is pumped into the zone to be cemented and allowed to set into a hard impermeable mass therein.
It is, therefore, a general object of the present invention to provide improved delayed thixotropic cement compositions and methods.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.